Extended Forecast Discussion NWS Weather Prediction Center College Park MD 513 PM EDT Mon Aug 14 2023 Valid 12Z Thu Aug 17 2023 - 12Z Mon Aug 21 2023 ...Heat wave to prevail across the southern Plains and build into the central U.S. by this coming weekend... ...Hazardous heat across parts of the West to wane after Thursday... ...Overview... The upper level pattern during the Medium Range period (Thursday-Monday) will trend more amplified with time. A vigorous shortwave will shift across the Great Lakes and Northeast later this week, with a potential embedded closed low tracking over or near northern New England around Saturday. Behind this feature an upper level ridge initially over the West should gradually trend stronger as it shifts into the Plains/east-central U.S., while upper troughing steadily descends over the West Coast. This trough may eject an upper low that will be offshore California through late this week. Hazardous heat across the South will expand northward into the central Plains by this weekend, while the West should trend cooler and wetter, possibly further enhanced by highly uncertain details involving eastern Pacific tropical development and associated moisture. Farther east, expect the evolving pattern to confine most rainfall to a brief episode over the Northeast Thursday-Friday as well as the Florida Peninsula and spreading westward along the Gulf Coast by the weekend or early next week. ...Guidance Evaluation/Predictability Assessment... The most noticeable trend in most guidance over the past day or so is toward greater amplitude overall. This includes a slower and more closed upper trough crossing New England/Southeastern Canada, as well as stronger/more amplified Plains/Mississippi Valley ridging during the weekend and early next week downstream from deeper troughing that settles over the West Coast. There are still individual solutions that differ significantly from this majority trend. The 00Z UKMET/CMC both strayed a lot faster with North Pacific/Aleutians flow that impacted the downstream pattern, but the 00Z CMCens mean looked much closer to the GFS/ECMWF and GEFS/ECens to add support to the latter majority. The new 12Z UKMET is quite progressive with energy crossing south-central Canada (interestingly, hinted at in the new 12Z ECMWF) but has adjusted closer to the GFS/ECMWF in principle farther west. On the other hand, the 12Z CMC is still somewhat progressive over the Pacific but is getting closer to the majority farther east. The 06Z compared better to latest clustering than the faster 12Z GFS for the New England/southeastern Canada upper low. Specifics of the West Coast pattern will be important for resolving how much moisture reaches the Southwest by the weekend and early next week, with a potential connection to an eastern Pacific tropical system for which the National Hurricane Center shows a high chance of formation within the coming 7 days. The 00Z ECMWF/ECMWF mean and 06Z GFS/GEFS mean form a majority track that is fairly well offshore the western Baja California coast but CMC runs and the 00Z GFS (with the new 12Z GFS between the two) are farther east and would provide a more direct feed of significant moisture into the Southwest. At the very least CMC runs are in the minority for exact upper level details along the California coast, tempering confidence in its tropical system track. Given the differences that appeared in the 00Z UKMET/CMC rather early (after Day 3 Thursday), the updated forecast started with an average of the 06Z GFS/00Z ECMWF early in the period and then transitioned to 50-60 percent total GEFS/ECens input by days 6-7 Sunday-Monday given the typical increase in detail uncertainty at that time. Regarding moisture/rainfall over the Southwest during the latter half of the period, latest trends have dramatically increased NBM guidance over the region. The manual forecast reflects a compromise between continuity and the NBM as a more measured increase for the Day 6 period, while the NBM was closer to a reasonable intermediate solution by Day 7. ...Weather/Hazards Highlights... A weakening frontal boundary along the Southeast Coast will allow anomalous moisture to linger over Florida where a Marginal Risk is in place over much of the Florida peninsula for the Day 4/Thursday Excessive Rainfall Outlook. To the north, showers and storms should accompany the vigorous shortwave/possible closing upper low and surface frontal system through the Great Lakes into the Northeast Thursday-Friday. Recent trends toward slower system progression (minus the 12Z GFS) along with above average soil moisture over the Northeast have led to a planned introduction of a Marginal Risk area over parts of the Northeast for the Day 5/Friday outlook. Beginning Thursday, but especially into this weekend, monsoonal moisture will increase across the Southwest and expand northward into the Great Basin/Rockies as upper troughing settles near the West Coast. The Day 4 ERO shows a Marginal Risk area over parts of the Four Corners region expanding over more of Utah/Arizona and western Colorado into the Day 5 period. Meanwhile the combination of anomalously high moisture and shortwave energy/upper height falls may also help to focus locally heavy convection over and east of the Sierra Nevada. Thus the plan is to introduce associated Marginal Risk areas for this activity during Days 4-5, with the Day 5 outlook confined more to just the Sierra Nevada as some of the moisture to the east lifts northward. For the Southwest later in the weekend and early next week, amounts are highly uncertain and dependent on potential tropical development and track in the eastern Pacific. The updated WPC QPF has trended somewhat wetter in light of the guidance average and the potential that moisture flow into the Southwest could be underdone in the 06Z GFS/00Z ECMWF even if their more western tropical system tracks verify, but the uncertainty seemed to merit lower amounts than the 13Z NBM especially around Day 6/Sunday. Continue to monitor upcoming forecasts as this appears to be a very sensitive evolution and rainfall in the wetter two-thirds of the spread could have significant effects on parts of the Southwest. Meanwhile, oppressive heat will continue through the period across the southern Plains to central Gulf Coast with numerous daily record highs and warm overnight lows likely. The combination of heat and humidity will also result in extremely dangerous heat indices in excess of 110F in many locations. The heat wave across the Northwest should continue into Thursday with temperatures around 10-20 degrees above average across the Northwest and around 5-15 degrees above average in the central valley of California into the Desert Southwest. This equates to highs in the 90s/100s and even localized 110s, which could set temperature records for daytime highs. Daily records for warm overnight lows are possible too. See Key Messages issued by WPC for additional information and graphics related to these two heat waves. The heat wave across the West should come to an end on Friday as upper troughing takes hold over the region this weekend, with a broad area of highs 5-15 degrees below normal by next Sunday-Monday. The eastward shift of the upper ridge will help to warm up the central U.S. by late this week. Daytime highs in the 90s to near 100 look to build into the central Plains to Midwest, with humidity levels bringing heat indices even higher. Hazardous heat is becoming more and more likely across this region starting this weekend and lasting into next week. The cooling trend over the West should lead to a broad area of highs 5-15 degrees below normal by next Sunday-Monday. Rausch/Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Central Great Basin, California, and the Southwest, Sun-Mon, Aug 20-Aug 21. - Hazardous heat across portions of the Northern/Central Great Basin, the Northern/Central Plains, the Northern/Central Rockies, California, the Pacific Northwest, and the Southwest, Thu, Aug 17. - Hazardous heat across portions of the Lower Mississippi Valley and the Southern Plains, Thu-Mon, Aug 17-Aug 21. - Hazardous heat across portions of the Great Plains, the Mississippi Valley, the Ohio/Tennessee Valley, and the Southeast, Fri-Mon, Aug 18-Aug 21. - Heavy rain across portions of mainland Alaska, Thu-Sat, Aug 17-Aug 19. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw